This invention relates generally to improvements in controlling growth processes of single crystal semiconductors for use in the manufacture of electronic components and, particularly, to methods of accurately controlling growth in a Czochralski crystal growth process for minimizing deviations in both growth rate and diameter, including determining steady-state growth rate and controlling pull rate around the steady-state growth rate, independent of process conditions such as sensed melt temperature.
Monocrystalline, or single crystal, silicon is the starting material in most processes for fabricating semiconductor electronic components. Crystal pulling machines employing the Czochralski process produce the majority of single crystal silicon. Briefly described, the Czochralski process involves melting a charge of high-purity polycrystalline silicon in a quartz crucible located in a specifically designed furnace. After the heated crucible melts the silicon charge, a crystal lifting mechanism lowers a seed crystal into contact with the molten silicon. The mechanism then withdraws the seed to pull a growing crystal from the silicon melt. A typical crystal lifting mechanism suspends the seed crystal from one end of a cable, the other end of which is wrapped around a drum. As the drum rotates, the seed crystal moves up or down depending on the direction that the drum is rotating.
After formation of a crystal neck, the growth process enlarges the diameter of the growing crystal by decreasing the pulling rate and/or the melt temperature until a desired diameter is reached. By controlling the pull rate and the melt temperature while compensating for the decreasing melt level, the main body of the crystal is grown so that it has an approximately constant diameter (i.e., it is generally cylindrical). Near the end of the growth process but before the crucible is emptied of molten silicon, the process gradually reduces the crystal diameter to form an end cone. Typically, the end cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt. During the growth process, the crucible rotates the melt in one direction and the crystal lifting mechanism rotates its pulling cable, or shaft, along with the seed and the crystal, in an opposite direction.
Although presently available Czochralski growth processes have been satisfactory for growing single crystal silicon useful in a wide variety of applications, further improvements are still desired. For example, conventional growth processes employ one of two main strategies for crystal growth control. First, standard control aims primarily to control diameter of the growing crystal. Second, xe2x80x9clocked seed liftxe2x80x9d control aims primarily to control growth rate. It is desired to minimize deviations in both diameter and growth rate simultaneously throughout the growth process to improve the shape and the quality of the as-grown crystal.
The problem of minimizing growth rate variations involves reliably measuring growth rate. In conventional growth processes, the pull rate, which is set by the seed lift, is a known parameter. The growth rate can be determined indirectly by examining the meniscus dynamics if the height of the meniscus at the crystal-melt interface is known as a function of time. In an ideal case, the ingot has a perfectly cylindrical shape and the pull rate is identical to the growth rate. Achieving relatively small deviations in crystal diameter is coupled to achieving relatively small deviations in growth rate from the prescribed value. Although attempts have been made using pyrometers or laser reflections from meniscus shape, a reliable system for measuring meniscus height is not available. Therefore, improvements in crystal growing processes are desired for determining growth rate without measuring meniscus height.
A controller is also desired for solving the problem of unstable crystal growth. In particular, such a controller is needed for maintaining a stable crystal growth for the duration of the growth process by minimizing diameter variations. Those skilled in the art recognize that pulling a crystal from a melt is an intrinsically unstable process. Without control, the crystal will grow in a conical, rather than generally cylindrical, shape. Active control loops are necessary to maintain the cylindrical shape, which means keeping the pull rate close to the steady-state growth rate. Perturbations in the steady-state growth rate can be time-dependent and can differ from run to run and/or puller to puller.
As described above, conventional growth processes use two main strategies for crystal growth control. The first primarily involves diameter control and the second primarily involves a xe2x80x9clocked seed liftxe2x80x9d for growth rate control. The presently available systems, however, do not provide simultaneous control of diameter and growth rate. For example, standard diameter control manages crystal growth by means of two PID (Proportional Integral Derivative) control loops acting on pull rate and heater power. The primary objective of the controller is to grow a crystal at diameter equal to a set value (given other process parameters such as crystal and crucible rotation or the presence of magnetic fields). The first PID control unit acts on pull rate to minimize the diameter error. As a secondary objective, the puller controller seeks to minimize the error between the average pull rate and the set value. This is accomplished by means of the second PID control loop acting on heater power. With the standard PID control system, diameter variation can be set to a tolerance of about 1 mm but relatively large pull rate deviations from the set value can still occur (e.g., even of the order of 30-40%).
The primary objective of conventional seed lift control systems is growth rate control. In this instance, pull rate is set to a constant prescribed value Vset. Diameter control is of secondary importance and is generally accomplished by means of a PID control unit acting directly on the heater power. Although this control strategy precisely controls pull rate relative to the set value, relatively large diameter deviations (e.g., up to xc2x15-10 mm) are possible. Moreover, the actual growth rate still deviates even though the pull rate equals Vset so the main goal of growth rate control is not completely achieved.
By properly tuning the controller, acceptable performance with respect to the growth rate may be possible with presently available control systems. In this instance, proper tuning means empirically setting the various controller coefficients by trial and error given a particular hot zone configuration and a set of process parameters. The set of coefficients is defined for different portions of the crystal length to compensate for changes in the thermal configuration caused by the batch nature of the process. Given the number of different zones taken into consideration and the type of controller, a very large number of coefficients (e.g., 50-100 or more) must be properly tuned to obtain acceptable growth characteristics. Any change in hot zone design or process conditions, such as crucible or crystal rotations or an applied magnetic field, can negatively impact the performance of the controller. More re-tuning iterations are then necessary to accommodate such changes. In addition, critical processes where minimum tolerances are set for diameter and growth rate deviations often must be personalized for each puller. Tuning and optimization of process control parameters, updating of control parameters to new hot zone configurations or process conditions, and management of puller dependent processes is very cumbersome and costly. Thus, a reduction in the number of iterations needed to achieve acceptable process performance or a reduction in resources dedicated to process maintenance and personalization will provide significant cost savings in producing superior silicon crystals.
For these reasons, an accurate and reliable apparatus and method for controlling silicon crystal growth to minimize both growth rate deviations and diameter deviations that is less costly and easier to tune is desired.
The invention meets the above needs and overcomes the deficiencies of the prior art by providing methods of minimizing both growth rate deviations and diameter deviations in a crystal ingot pulled from a melt according to the Czochralski process. Among the several objects of the invention may be noted the provision of such methods that reduce the number of control parameters; the provision of such methods that permit determining the control parameters independently of crystal position, hot zone configuration, process conditions and/or puller; the provision of such methods that permit determining the control parameters from observation of diameter evolution; the provision of such methods that predict desired interventions on the pull rate and the power; the provision of such methods that permit tuning the control parameters during the run; the provision of such methods that permit deriving the meniscus height dynamics from measured radius dynamics; the provision of such methods that permit determining meniscus height measurements as a function of the derived meniscus height dynamics; the provision of such methods that may be incorporated into existing crystal pulling devices; and the provision of such methods that can be carried out efficiently and economically.
Briefly described, a method embodying aspects of the invention is for use with a crystal puller for growing a monocrystalline semiconductor crystal according to the Czochralski process. The crystal puller has a heated crucible containing a semiconductor melt from which the crystal is grown. The crystal puller also has a heater supplied by a power supply for heating the crucible. The crystal is grown on a seed crystal pulled from the melt at a pull rate Vp. The method includes defining an initial interval of time for observing growth of the crystal being pulled from the melt and determining variations in crystal diameter occurring during the observation interval. Based on the variations in the crystal diameter, the method estimates a current growth rate Vg at which the crystal is being grown. The method also includes estimating a current steady-state value of the growth rate Vgs as a function of the estimated growth rate Vgf at the end of the observation interval and determining a pull rate parameter and a heater power parameter as a function of the estimated steady-state growth rate Vgs. The pull rate parameter is representative of an incremental change in the pull rate Vp to effect a desired change in the diameter of the crystal toward a target diameter and the heater power parameter is representative of an incremental change in the power supplied to the heater to effect a desired change in the growth rate of the crystal toward a target growth rate. According to the invention, the parameters are determined independently of a temperature condition sensed during pulling. The method further includes adjusting the pull rate Vp according to the pull rate parameter and adjusting the power supplied to the heater by the power supply according to the heater power parameter. This minimizes variations in both crystal diameter and growth rate during subsequent growth of the crystal following the initial observation interval.
Another embodiment of the invention is directed to a control method for use with a crystal puller for growing a monocrystalline semiconductor crystal according to the Czochralski process. The crystal puller has a heated crucible containing a semiconductor melt from which the crystal is grown. The crystal puller also has a heater supplied by a power supply for heating the crucible. The crystal is grown on a seed crystal pulled from the melt at a pull rate Vp. The melt has a surface with a meniscus adjacent the crystal as the crystal is pulled from the melt. The method includes defining an initial interval of time for observing growth of the crystal being pulled from the melt and determining variations in crystal diameter occurring during the observation interval. In addition, the method includes defining a function r(t) based on the variations in crystal diameter occurring during the observation interval. The function r(t) represents radius variations and is a function of current values of crystal radius r, meniscus height h and growth rate Vg with respect to time. The method then performs a best fit routine on the function r(t) to deduce the current values of crystal radius rf, meniscus height hf and growth rate Vgf at the end of the observation interval. In turn, a current steady-state value of the growth rate Vgs is calculated as a function of the deduced current values of crystal radius rf, meniscus height hf and growth rate Vgf at the end of the observation interval and independent of melt temperature and meniscus height sensed during pulling. The method further includes controlling the crystal puller as function of current steady-state growth rate Vgs to minimize variations in both crystal diameter and growth rate during subsequent growth of the crystal.
In yet another embodiment, a method of the invention is for use with a crystal puller for growing a monocrystalline semiconductor crystal according to the Czochralski process. The crystal puller has a heated crucible containing a semiconductor melt from which the crystal is grown. The crystal puller also has a heater supplied by a power supply for heating the crucible. The crystal is grown on a seed crystal pulled from the melt at a pull rate Vp. The method includes defining an initial interval of time for observing growth of the crystal being pulled from the melt and determining variations in crystal diameter occurring during the observation interval. Based on the variations in the crystal diameter, the method estimates a current growth rate Vg at which the crystal is being grown. The method also includes estimating a current steady-state value of the growth rate Vgs as a function of the estimated growth rate Vgf at the end of the observation interval and independent of meniscus height measured during pulling. In turn, the method determines a pull rate parameter and a heater power parameter as a function of the estimated steady-state growth rate Vgs. The pull rate parameter is representative of an incremental change in the pull rate Vp to effect a desired change in the diameter of the crystal toward a target diameter and the heater power parameter is representative of an incremental change in the power supplied to the heater to effect a desired change in the growth rate of the crystal toward a target growth rate. The method further includes adjusting the pull rate Vp according to the pull rate parameter and adjusting the power supplied to the heater by the power supply according to the heater power parameter. This minimizes variations in both crystal diameter and growth rate during subsequent growth of the crystal following the initial observation interval.
Alternatively, the invention may comprise various other methods and systems.
Other objects and features will be in part apparent and in part pointed out hereinafter.